A current transformer (CT) is a sensor that measures electrical current by sensing the magnetic field around a conductor and converting it into a proportional signal that a meter, controller, or energy management system can read. In EV charging installations, CTs are widely used for site load monitoring, dynamic load balancing, and protecting the main electrical connection from overload.
What Is a Current Transformer (CT)?
A CT is an instrument transformer designed to measure current safely without putting measurement electronics directly in the high-current path. Common CT forms include:
– Split-core (clamp-on) CTs that can be installed without disconnecting cables
– Solid-core CTs that require a conductor to pass through the CT window during installation
CTs are typically installed in electrical panels, switchboards, or feeder cabinets and connected to a meter or controller.
Why CTs Matter for EV Charging
EV charging adds significant electrical load, often in buildings that already have variable demand. CTs matter because they enable:
– Real-time monitoring of total site current draw
– Load balancing that automatically adjusts charger current limits to stay within site capacity
– Avoiding trips of main breakers and reducing downtime risk
– Better utilization of existing grid connection without expensive upgrades
– More predictable operation under connection tariffs and contracted capacity limits
Without accurate CT measurement, load management becomes unreliable and can cause nuisance trips or underuse of available capacity.
How a CT Works
A CT measures current indirectly:
– Current flowing through a conductor generates a magnetic field
– The CT senses that field and induces a proportional current or voltage on its secondary side
– The meter/controller converts that signal into an actual current value using the CT ratio
This provides isolation and allows safe measurement of very large currents.
Where CTs Are Installed in Charging Projects
CT placement depends on the control objective:
Main Supply Monitoring
– CTs on the building/site incoming phases measure total import
– Enables site-wide power caps and dynamic allocation across chargers
Charger Feeder or Sub-Panel Monitoring
– CTs measure a specific feeder serving a charger group
– Supports cluster-level management for large car parks and depots
Per-Phase Monitoring
– CTs on each phase detect phase imbalance
– Enables phase-aware charging strategies and more stable capacity use
CTs and Dynamic Load Balancing
With CT input, a load management controller can:
– Reduce charger current when building load rises
– Increase charger current when building load drops
– Share available power across multiple charge points
For AC charging, current limits are communicated to the vehicle through control pilot (CP) PWM signaling.
Accuracy and Installation Quality
Correct CT selection and installation are essential:
– Choose the correct CT ratio for expected current range
– Install CTs on the correct conductors and map phases correctly
– Maintain correct polarity/orientation to avoid inverted readings
– Use proper wiring and secure terminations to prevent intermittent signals
Incorrect CT wiring can cause unstable power limits or false readings that impact charging performance.
Common Pitfalls
– CTs installed on the wrong cables or phases mixed up
– Incorrect CT ratio leading to poor measurement accuracy
– Reversed polarity resulting in negative or unstable load readings
– CT saturation on peaks, causing under-reporting at high loads
– Skipping validation during commissioning, so load balancing behaves unpredictably
– Missing CT placement and phase mapping in commissioning documentation
Related Glossary Terms
Load Balancing
Phase-Aware Charging
Control Pilot (CP)
Electrical Panels
Circuit Breakers
Connection Tariffs
Charging Capacity Planning
Commissioning Documentation
Uptime